Sift the flour into a medium bowl. Whisk the milk and eggs together in a medium jug; add liquid to flour and whisk until smooth.
Brush a small non-stick frying pan with a little of the melted butter. Heat over moderate heat. Pour 1/4 cup of the crepe batter into pan. Cook for 1-2 minutes or until golden underneath and bubbles appear on the surface. Turn, then cook for 1-2 minutes more or until golden and cooked. Transfer to serving plate. Cover to keep warm. Repeat with remaining butter and crepe batter to make 8 crepes.
Fold crepes in half. Spread with Nutella and top with banana slices. Fold in half again to enclose filling. Serve at once.